> > type planet = {
> >   mutable x : float;  mutable y : float;  mutable z : float;
> >   mutable vx: float;  mutable vy: float;  mutable vz: float;
> >   mass : float;
> > }
> 
> Advice from old FORTRAN hacker:
> 
> This is a bad data structure for Ocaml.

Advice from an old Caml hacker: this is an excellent data structure
for OCaml.

Second advice from an old Caml hacker: don't trust John Skaller's
technical advice too much.  John often has preconceptions about what
Caml is, or does, or "must" do (in his mind), which bear
no connections with reality.  That would be a minor annoyance except
for the very authoritative, expert-like tone in which these
preconceptions are assessed, which can mislead non-expert readers.

- Xavier Leroy